It is known and widely recognized that the processing of plastics containing moisture severely affects the quality of the processed material. The surface appearance is affected and the accuracy and strength of the molded product decreases due to the presence of moisture in the plastic pellets or powder being processed. The less moisture contained in the plastic starting material the better the final product is the case for most moldable plastic material. Specifically, engineering plastics such as nylon, polycarbonate, ABS, and polyethylenepterathylate (PET) are greatly affected by the presence of moisture during the molding process. For example, water vapor content should be below 50 ppm for molding of PET bottles, and below 100 ppm for molding polycarbonate compact discs.
Conventionally, a heated, dried air is circulated through the wet plastic material. The hot dry air is generated by heaters and disicant materials. The efficiency of heating air by means of heat exchangers is low, as is the efficiency of heating plastic materials with air. For example, the proper drying of PET with air requires 5-6 hours. The plastic materials dried by the conventional methods are heated from the outside. Since plastics are good heat insulators, drying times are long.
What is needed is a method which shortens drying time, enables more uniform drying of pelletized or powdered plastic material, and improves the efficiency of the drying process.